Method and device for doping, diffusion and oxidation of silicon wafers under reduced pressure

ABSTRACT

Method and device for doping or diffusion, or oxidation of silicon wafers ( 4 ), the wafers being introduced into the chamber ( 2 ) of an oven ( 1 ) wherein is introduced at least a gas for performing the doping or diffusion or oxidation process. The method comprises simultaneously with the introduction and passage of gas into the chamber ( 2 ) of the oven ( 1 ), continuously subjecting the latter to a depression of constant value. The device comprises an oven ( 1 ) provided with a chamber ( 2 ) wherein are introduced the wafers, the oven including at least an inlet tube ( 5   a,    5   b,    5   c ) for introducing at least a gas into the chamber ( 2 ) to carry out the processes and at least an outlet tube ( 6 ) for extracting the gas whereto is connected a suction unit ( 7 ) for generating in the chamber ( 2 ) a constant and controlled depression.

[0001] The present invention relates to N and P type doping, diffusion and oxidation of silicon wafers in the process for production of semiconductors.

[0002] In the process for the production of integrated circuits for electronics, the techniques of fabricating semiconductors are based principally on the intrinsic modification of the atoms of the silicon material. The insertion in the silicon lattice of elements with a surplus of electrons, of the so-called N type, and a deficit of electrons, of the so-called P type, is defined by the term doping of the N type or the P type.

[0003] To render silicon semiconductive, it is necessary to add to it so-called doping elements.

[0004] The known doping elements are for example phosphorus, arsenic, antimony, boron, gallium or aluminum. In the processes most often used for doping with phosphorus or boron for example, the silicon wafers are introduced into an oven and brought to a temperature comprised generally between 800° C. and 1200° C. This temperature is necessary to permit a superficial concentration of the phosphorus or the boron on the surface of the silicon wafer.

[0005] The sources of solid, liquid or gaseous dopants are vaporized in the chamber of the oven from a carrier gas in contact with the source of dopants. The carrier gas plays the role of accelerating the doping of the dopant gas on the surface of the silicon wafer. This type of deposit takes place in the oven at atmospheric pressure. To complete the distribution between the wafers, the carrier gas must be supplied at a high gas vector flow rate, which gives rise to a large consumption of gas.

[0006] The horizontal or vertical ovens used generally have a chamber of tubular shape in which the silicon wafers are disposed on a quartz or silicon carbide support. One of the ends of the tubular chamber is provided with a door to permit the introduction of the wafers. The other end is closed by a non-removal end wall. The doping gas, the reacting gas and the vector gas are generally injected through an opening provided in the end wall of the tubular chamber.

[0007] To obtain good uniformity of treatment, it is necessary to limit the number of wafers in the oven, generally not more than 50, and to leave a relatively large space between each wafer.

[0008] The use of a vector gas also gives rise to problems of substantial condensation of acid in the oven and the environment. This acid deposition from the dopant will have repercussions on the repetitivity of the treatment of the wafer but also on the different external constituent elements of the oven, which requires frequent disassembly for cleaning of the oven and particularly of the quartz tube comprising the chamber of this latter.

[0009] On the other hand, quartz diffusers, to improve the result, are necessary, increasing the costs of use and control of the quartz portions. The returned operation after cleaning requires preliminary tests giving rise to loss of productive capacity. Other phenomena such as the lack of uniformity on the wafer and on the load and from load to load, a memory effect of doping, parasitic inclusions of dopants in the crystallographic lattice of silicon because of excess oxygen, modifications of lifetime of the minority carriers, the limitation of the diameter of the wafers, are also factors requiring a more burdensome choice of equipment.

[0010] The present invention has for its object to solve the problems mentioned above by providing a new process of doping, diffusion and oxidation.

[0011] The process according to the invention is essentially characterized in that it consists in conjointly with the introduction of the gas into the oven chamber, subjecting this latter to reduced pressure. This effect permits increasing the speed of the gas in the chamber and renders no longer necessary the use of large quantities of vector gases. Only the reactive gases are present in the silicon doping chamber.

[0012] According to another characteristic of the invention, the value of the under pressure is comprised between 100 and 800 millibars. This range of values is compatible with relatively simple means for the creation of the under pressure.

[0013] The invention also relates to a technique for injection of oxygen which permits dosing with precision the quantity of oxygen to limit the speed of cracking of the dopant, which is to say is molecular decomposition.

[0014] The invention also relates to the device permitting carrying out an operation of doping, diffusion or oxidation of the wafers. The device comprises an oven provided with a chamber hermetically closed by a door, into which chamber are introduced the wafers, said oven comprising at least one injection tube for at least one gas into the chamber to carry out one of the mentioned operations.

[0015] The device as defined is characterized essentially in that it comprises moreover at least one gas extraction tube, an aspiration means connected to the gas extraction tube, adapted to create in the chamber a constant and controlled under pressure.

[0016] Thus, the use of large quantities of vector gas of the nitrogen type is no longer necessary because of the under pressure that is created. Moreover, the peripheral condensation of acid is greatly reduced, the acid being in large part drawn into the aspiration means. The residual condensation will be itself recovered by added devices such as tiles or the like.

[0017] The present invention also has for its object a device for doping, diffusion or oxidation of silicon wafers, comprising an oven hermetically closed by a door, into which chamber are introduced said wafers, said oven comprising at least one introduction tube for at least one gas into the chamber to carry out the mentioned operations.

[0018] Other advantages and characteristics of the invention will become apparent from the reading of the description of a preferred embodiment, given by way of non-limiting example and illustrated by the accompanying drawings, in which:

[0019]FIG. 1 shows schematically the device for doping, diffusion or oxidation of silicon wafers according to the present invention,

[0020]FIG. 2 shows in longitudinal cross-section a device for doping, diffusion or oxidation, according to the invention.

[0021] As shown, the device for doping, diffusion or oxidation of silicon wafers according to the invention comprises an oven 1 provided with a sealed chamber 2 associated with heating means 3, and into which are introduced silicon wafers that are to be subjected to the mentioned operations. The oven 1 comprises at least one introduction tube 5 a, 5 b, 5 c, for at least one gas into the chamber 2, to achieve the mentioned operations and comprises moreover at least one gas removal tube 6, and suction means 7 connected to said gas removal tube 6, adapted to create in the chamber a constant and controlled under pressure, this suction unit being located at a distance from the oven 1 in a temperate region.

[0022] The chamber 2, for example of tubular shape, is hermetically closed by a fixed end wall 2 a and by a door 2 b installed at its ends, the door, in the closed position, and the end wall being perpendicular to the longitudinal axis of symmetry of the tubular chamber.

[0023] The heating means 3, constituted for example by electrical resistances, are distributed or wound about the tubular chamber and are disposed spaced from this latter or else in contact therewith.

[0024] Preferably, the tubular chamber 2 has a cross-section in the form of a circular crown. This arrangement gives the wall of the chamber good mechanical resistance to atmospheric pressure when the internal volume of the chamber is subject to under pressure.

[0025] The door 2 b is secured to a mounting slidably engaged on a suitable support. The opening and closing movements of the door take place along the longitudinal axis of the tubular chamber. With the door is associated a cover extending outwardly of the chamber in which is mounted the shaft of a blade for loading wafers. This blade receives wafer supports. Preferably with this cover is associated an means for adjusting the inclination of the blade and said cover is mounted with the possibility of pivotal movement about a horizontal axis normal to the longitudinal axis of the blade. By action on the adjustment means, the blade, under vacuum, is more or less upwardly inclined, such that when the latter is loaded it can substantially come into position substantially along a horizontal plane. This adjustment can be carried out with a weight on the blade, the weight of this load corresponding to the weight of the load to be treated.

[0026] Preferably, the door 2 b of the oven chamber is constituted of opaque quartz or is opacified and has a suitable sealing joint made of a material based on a fluorinated elastomer. Such a material is particularly known under the commercial name “VITON”. Such a material is made by the DUPONT DE NEMOURS company.

[0027] The opaque quartz constitutes an effective barrier to infrared, burning of the sealing joint is thus avoided. So as further to reinforce the protection of the sealing joint, the terminal portion of the tubular chamber 2 (the one adapted to receive the door 2 b) is made of opaque quartz. In practice, this opaque portion is assembled with the transparent door of the chamber by any means known to those skilled in the art. In the preferred embodiment, this assembly is made by welding. Again so as to reinforce the thermal protection of the joint, a thermal screen is disposed in the chamber facing the door.

[0028] The door, by its sealing joint, is applied against an annular edge of the opaque terminal portion of the tubular chamber, this annular edge being also opaque. The annular surface of this edge, facing the joint of the door, constitutes a joint plane and receives the joint under pressure. This joint is preferably mounted in an annular throat hollowed out of the door 2 b. To absorb defects of parallelism between the door and the joint plane, said door is mounted floatingly on the mounting, which is to say with limited latitude of movement about two secant and orthogonal axes parallel to the plane of the joint and along an axis normal to this joint plane. Preferably, between the door and its mounting are disposed one or several resilient members and the cover comprises an axially deformable portion.

[0029] To resist heat, the walls of the oven, namely, the tubular wall of the chamber 2 and the terminal wall 2 a, are constituted of quartz. The tubular chamber could be provided with an internal covering of silicon carbide of tubular shape. This covering resists radial deformation inwardly of the tubular chamber 2, particularly when this latter is subjected to high temperature.

[0030] The path of the gas in the chamber 2 of the oven 1 is established from an upstream region 8 toward a downstream region 9 located at a distance from the former, the silicon wafers 4 being disposed between these two regions along the path of the gases. These wafers 4 are disposed in the chamber 2 of the oven transversely to the direction of flow of the gases in this latter. Thus there is created a relatively great pressure drop thanks to which the gases can sweep the surfaces to be treated of the wafers.

[0031] The wafers 4 are installed on a removable support 10 introduced into the chamber 2 of the oven. This support is made of a material adapted to resist both the heat as well as the corrosive power of the gases. Thus this support could be constituted of silicon carbide or else of quartz. The support and the wafers, after positioning in the chamber, are disposed at a spacing from the end wall 2 a and from the door 2 b, which provide the upstream region 8, this latter being located between the end wall 2 a and the position of the wafer support, and the downstream region 9 located between the door 2 b and said position, the gas flow in said chamber 2 being established between one and the other zone and the region of the chamber corresponding to the position of the silicon wafer support being the hottest zone of said chamber 2.

[0032] The introduction tube or tubes 5 a, 5 b, 5 c of the gas or gases in the chamber 2 of the oven pass through the end wall 2 a of this latter or the door 2 b and flow into one or the other upstream region 8 or downstream region 9 of the withdrawal tube 6, gases passing through the end wall of the oven 2 a or the door 2 b and flowing into the other upstream or downstream zone 8 or 9 of the chamber 2. According to the preferred embodiment, the tube or tubes 5 a, 5 b, 5 c for introduction of gas into the chamber 2 and the withdrawal tube 6 of the gas, pass through the end wall 2 a of the chamber, and the introduction tubes 5 a, 5 b, 5 c of the gas open into the upstream region 8 of the chamber whilst the withdrawal tube 9 for the gases opens into the downstream region 9 of said chamber 2.

[0033] One of the tubes 5 c for introduction of gas into the chamber 2 receives a reactive gas of the type of oxygen. Preferably, this tube 5 c opens into the warmest region of this latter, nearest the position occupied by the wafer support, to avoid the anticipated cracking of the doping gas.

[0034] As mentioned above, the device according to the invention is provided with a unit 7 for sucking out gases from the chamber 2, adapted to create continuously an under pressure of constant and perfectly controlled value. This under pressure has the object of increasing the speed of the gases in the chamber and avoiding the use of vector gases.

[0035] According to the preferred embodiment, the suction means 7 comprises particularly a suction pump 11 preferably of the membrane type. This pump 11 at least for its members in contact with the gas, will be made of a material adapted to resist the corrosion of said gases. There could be used a material constituted of tetrafluoroethylene. Such a material is known under the commercial name of “TEFLON”.

[0036] Again according to the preferred embodiment, the suction unit 7 comprises control and regulation members of the under pressure in the oven chamber, these control and regulation members being in communication through conduits on the one hand with the suction tube of the pump and on the other hand with the removal tube from the oven.

[0037] The control and regulation members comprise particularly a over pressure or ballast, constituted by a cylindrical chamber connected by conduits, on the one hand to the suction tubing of the pump and on the other hand to the tubing 6 for withdrawal from the oven, said overfilling capacity 12 being moreover connected to a source under pressure of over pressure gas, controlled by means of a control valve 13, directed to open and close by a circuit for measuring and controlling the dynamic under pressure in the chamber 2 of the oven 1.

[0038] This measuring and control circuit of the under pressure comprises a pressure detector 14 disposed outside the chamber, in the connection conduit extending between the over pressure capacity and the withdrawal tube of the oven or else to the over pressure capacity. This pressure detector 14 is adapted to produce a signal of an intensity proportional to the value of the under pressure, this signal being applied to the inlet of the compressor 15 which compares the value of this signal with a reference value. This comparator is connected by a power circuit to the control valve 13 for, as a function of the spacing between the value of the under pressure and the reference value, adjusting the degree of opening of the valve member of the control valve 13 and hence the flow rate of the over pressure gas or controlling the closing of this closure member of the control valve, which permits by over pressure more or less pronounced, from the pump or by the absence of over pressure, adjusting at each instance the value of the under pressure in the chamber 2 of the oven. Stated otherwise, the suction capacity of the pump being constant, the increase of flow rate of the over pressure gas is accompanied by a decrease of the flow rate of the gases extracted from the chamber of the oven, and vice versa.

[0039] The over pressure capacity 12 receives over pressure gas at ambient temperature and the hot gas is withdrawn from the oven chamber. The mixture of the gases in this over pressure capacity gives rise to condensation which is recovered by this capacity. Thus this condensation can in no way disturb the operation of the pump 11. The over pressure capacity will be removable, particularly for extracting the various condensations.

[0040] It will be seen that the degree of under pressure in the oven chamber and the control of this latter, take place outside of the oven in relatively cool and temperate regions.

[0041] The gases rejected by the pump are then neutralized with the help of any appropriate means.

[0042] The advantages of the invention are as follows:

[0043] compatibility with atmospheric technology,

[0044] uniformity of the treatment of each wafer, of each load (assembly of plates on a support), and from load to load,

[0045] repetitivity,

[0046] reproductivity after stopping,

[0047] suppression of the memory effect,

[0048] suppression of overdoses of dopants,

[0049] no limitation on the diameter of the wafers,

[0050] considerable reduction of maintenance,

[0051] reduction of costs,

[0052] peripheral cleanliness,

[0053] frequency of cleaning greatly decreased.

[0054] It follows that the teachings of the present invention are equally applicable to an oven of the horizontal type as to an oven of the vertical type. It is quite evident that the present invention can be the subject of any arrangement and modification within the field of technical equivalents, without thereby departing from the scope of the present application. 

1. Process for doping or diffusion or oxidation of silicon wafers (4), said wafers being introduced into the chamber (2) of an oven (1) into which is introduced at least one gas to accomplish the operation of doping or diffusion or oxidation, characterized in that it consists simultaneously to the introduction and to the passage of the gas into the chamber (2) of the oven (1), in subjecting this latter, continuously, to an under pressure of constant value.
 2. Process for doping or diffusion or oxidation, according to claim 1, characterized in that the value of the under pressure in the oven chamber is comprised between 100 and 800 millibars.
 3. Device for doping, diffusion or oxidation of silicon wafers (4), comprising an oven (1) provided with a chamber (2) hermetically sealed by a door (2 b), into which chamber are introduced said wafers, said oven comprising at least one inlet tube (5 a, 5 b, 5 c) for at least one gas into the chamber (2) to perform the mentioned operations, characterized in that it moreover comprises at least one tube (6) for removal of the gas, and a suction unit (7) connected to said gas extraction tube (6) adapted to create in the chamber (2) a constant and controlled under pressure.
 4. Device according to claim 3, characterized in that the suction means comprises particularly a suction pump (11).
 5. Device according to claim 4, characterized in that the pump (11) is of the membrane type.
 6. Device according to claim 4 or claim 5, characterized in that the pump (11) at least as to its parts in contact with the gas, is made of a material adapted to resist corrosion by said gas.
 7. Device according to claim 6, characterized in that this material is constituted of tetrafluoroethylene.
 8. Device according to claim 4, characterized in that the suction means (7) comprise control and regulation members for the under pressure in the furnace chamber, these control and regulation members being in communication by conduits with on the one hand the suction tube of the pump and on the other hand with the removal tube (6) of the oven.
 9. Device according to claim 4, characterized in that the control and regulation members comprise particularly an over pressure or ballast capacity (12), connected by conduits, on the one hand to the suction tube of the pump (11) and on the other hand to the withdrawal tube (6) of the gases from the oven chamber, said over pressure capacity (12) being moreover connected to a source of gas under pressure controlled by means of a control valve (13) for the opening and closing by a measurement and control circuit of the under pressure in the chamber (2) of the oven (1).
 10. Device according to claim 9, characterized in that the measuring and control circuit for the under pressure comprises a pressure detector (14) disposed outside the chamber (2), in the connection conduit extending between the over pressure capacity and the removal tube from the oven, said pressure detector (14) being adapted to produce a signal of an intensity proportional to the value of the under pressure, said signal being applied to the inlet of a comparator (15) which compares the value of this signal to a reference signal, said comparator being connected by a power circuit to the control valve for, as a function of the spacing between the under pressure value and the reference value, adjusting the degree of opening of the valve closure (13) or controlling the closing of the control valve closure (13) which permits by more or less pronounced over pressure of the pump (11) or by the absence of over pressure to adjust at each instant the value of the under pressure in the chamber (2) of the oven.
 11. Device according to any one of claims 3 to 10, characterized in that the silicon wafers (4) are disposed in the oven chamber (2) transversely to the direction of flow of the gases in this latter.
 12. Device according to any one of claims 3 to 11, characterized in that the wafers (4) are installed on a removable support (10) introduced into the chamber (2) of the oven, said support and said wafers after positioning in the chamber being disposed spaced from the end wall (2 a) and from the door (2 b), which provides an upstream region (8) located between the end wall (2 a) and the position of the wafer support and a downstream region (9) located between the door (2 b) and said emplacement, the gas flow in said chamber being established between one and the other region and the region of the chamber corresponding to the position of the silicon wafer support being the hottest region in said chamber.
 13. Device according to claim 12, characterized in that the tube or tubes (5 a, 5 b, 5 c) for introduction of the gas or gases into the oven chamber pass through the end wall (2) of the chamber of the oven or the door (2 b) and open into one or the other upstream or downstream zone (9) and that the removal tube (6) for the gases passes through the end wall (2 a) of the oven or the door (2 b) and opens into the other downstream or upstream region of the chamber (2).
 14. Device according to claim 13, characterized in that the introduction tube or tubes (5 a, 5 b, 5 c) for the gas into the chamber (2) and the removal tube (6) of the gas pass through the end wall (2 a) of the chamber, that the tubes (5 a, 5 b, 5 c) for introduction of the gas open into the upstream region (8) of the chamber and that the tube (6) for withdrawal of the gas opens into the downstream region (9) of said chamber (2).
 15. Device according to claim 13 or claim 14, characterized in that the introduction tube (5 a, 5 b, 5 c) of the reactive gas into the chamber (2) opens into the warmest zone of this latter, nearest the position occupied by the wafer support, to avoid anticipated cracking of the dopant gas.
 16. Device according to any one of claims 3 to 15, characterized in that the chamber (2) of the oven (1) is of tubular shape, the door (2 b) being disposed at one of the ends of said tubular chamber, and the end wall (2 a) at the other end, the door (2 b), in the closed position, and the end wall (2 a) being perpendicular to the longitudinal axis of symmetry of the tubular chamber.
 17. Device according to claim 16, characterized in that the tubular chamber (2) has a cross-section in the shape of a circular crown.
 18. Device according to claim 16 or 17, characterized in that the heating means (3) of the oven, constituted by electrical resistances, are distributed or wound about the tubular chamber (2).
 19. Device according to any one of claims 3 to 18, characterized in that the door (2 b) is constituted of opaque quartz and has a sealing joint.
 20. Device according to any one of claims 3 to 19, characterized in that the walls of the oven are constituted of quartz.
 21. Device according to any one of claims 3 to 20, characterized in that the end portion of the oven chamber is made of opaque quartz.
 22. Device according to any one of claims 3 to 21, characterized in that the door is secured to a mounting engaged slidably on a support and that said door is mounted floatingly on the mounting, which is to say with a limited latitude of movement about two secant and orthogonal axes parallel to the joint plane of the door with the chamber and along an axis normal to this joint plane.
 23. Device according to any one of claims 3 to 22, characterized in that the tubular chamber receives an internal cladding of silicon carbide.
 24. Device according to any one of claims 3 to 23, characterized in that the tubular chamber facing the door receives a thermal screen. 